Electrospray mass spectrometer and ion source

ABSTRACT

An inexpensive electrospray mass spectrometer capable of performing measurements consecutively from the ESI mode to the cold-spray ionization mode and vice versa. The electrospray mass spectrometer has an electrospray ion source, a nebulization nozzle, and a sampling orifice. The axes of the nozzle and orifice intersect each other. The instrument has a movable cold-spray desolvation chamber. In the electrospray ionization mode, the desolvation chamber is placed off the axis of the nebulization nozzle. In the cold-spray ionization mode, the desolvation chamber is set on the axis of the nebulization nozzle.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electrospray mass spectrometer andan ion source therefor.

2. Description of Related Art

An electrospray mass spectrometer using a soft ionization method hasbeen proposed. In particular, a sample in solution is pumped from aliquid chromatograph (LC) or held in a solution reservoir. The sample issent to a metallic capillary and drawn into it by pressure applied by anLC pump or by capillarity. A high voltage of several kilovolts isapplied between the capillary and a counter electrode of the massspectrometer to produce an electric field between them. The sample insolution in the capillary is electrostatically sprayed as chargeddroplets by the action of the electric field. The droplets are dried orcooled and guided into the mass spectrometer where they are analyzed.

This electrospray mass spectrometer provides a very soft ionizationmethod in that neither application of heat nor bombardment ofhigh-energy particles is used in ionizing sample molecules. Therefore,polar biopolymers, such as peptides, proteins, and nucleic acids, can beeasily ionized as multiply charged ions almost non-destructively.Furthermore, they are multiply charged ions and so those which havemolecular weights of more than 10,000 can be measured with a relativelysmall mass spectrometer. In this way, this instrument has excellentfeatures.

Analytical methods of electrospray mass spectrometry include ananalytical method using an ordinary ESI (electrospray ionization) ionsource (for example, Japanese Patent Laid-Open No. 2002-15697) and ananalytical method using a cold-spray ion source (for example, JapanesePatent Laid-Open No. 2000-285847). In the former method, charged liquiddroplets are electrostatically sprayed. Solvent molecules form clustersaround sample molecules in this spray of droplets. The solvent moleculesare vaporized by heating. In the latter method, liquid droplets areformed by electrostatic nebulization or by nebulization withoutapplication of a voltage. The droplets are cooled to minimize removal ofthe solvent. Molecular ions with solvent molecules attached areproduced. The solvent droplets are removed in a low-temperaturedesolvation chamber. These two methods have used with their respectivededicated ion sources. Therefore, measurements cannot be performedconsecutively moving from the ESI mode to the cold-spray ionization modeand vice versa. Hence, two ion sources must be used. This increases thecost of the equipment. In addition, the analysis is complicated.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an inexpensive massspectrometer instrument capable of performing measurements consecutivelymoving from the ESI mode to the cold-spray ionization mode and viceversa.

This object is achieved by a mass spectrometer fitted with anelectrospray ion source having a nebulization nozzle and a samplingorifice. The axis of the nozzle and the axis of the orifice intersecteach other. This spectrometer is further fitted with a movablecold-spray desolvation chamber. This movable desolvation chamber can bemoved off the axis of the nebulization nozzle in the electrosprayionization mode and can be set on the axis of the nebulization nozzle inthe cold-spray ionization mode.

In one feature of the present invention, the nebulization nozzle has acapillary for guiding a sample solution supplied from a sample inletport and a guide pipe coaxially surrounding the outer surface of thecapillary. The guide pipe guides a nebulizing gas introduced from a gasinlet port.

In another feature of the present invention, the temperature of thenebulizing gas is set to room temperature in the electrospray ionizationmode and from room temperature to about −50° C. in the cold-sprayionization mode.

In a further feature of the present invention, the nebulization nozzleis inserted substantially coaxially in the cylindrical desolvationchamber and opens into this chamber. A heater for heating is buried inthe walls of this chamber. This cylindrical desolvation chamber has agas inlet port for introducing a heating-and-drying gas.

In still another feature of the present invention, the potentialdifference between the nebulization nozzle and the sampling orifice is 1to 3 kV, and the potential difference between the walls of thecylindrical desolvation chamber and the sampling orifice is from zero tohundreds of volts.

In yet another feature of the present invention, where ions to beobserved are positive ions, the potential at the sampling orifice is setlower. Conversely, where ions to be observed are negative ions, thepotential at the sampling orifice is set higher.

In an additional feature of the present invention, the flow rate of thesample solution is 1 to 1,000 microliters/minute when a mixture ofdroplets of the sample in nebulizing gas is electrostatically sprayedfrom the nebulization nozzle.

In another feature of the prevent invention, a heating-and-drying gas isintroduced from the gas inlet port in the electrospray ionization mode.This heating-and-drying gas and heating performed by a heater buried inthe walls of the desolvation chamber cooperate to dry and desolvate theliquid droplets.

In another feature of the present invention, the heating temperature ofthe cylindrical desolvation chamber achieved by the heater isapproximately +100 to 300° C.

In another feature of the present invention, the heating-and-drying gashas a temperature of approximately +100 to 300° C.

In another feature of the present invention, the supply of theheating-and-drying gas into the cylindrical desolvation chamber from thegas inlet port is discontinued in the cold-spray ionization mode. Also,the electric power supplied to the heater buried in the inner wall ofthe desolvation chamber is cut off. Multiply charged molecular ions withsolvent molecules attached are produced.

In another feature of the present invention, a cooled gas is suppliedfrom the gas inlet port into the cylindrical desolvation chamber in thecold-spray ionization mode.

In another feature of the present invention, the temperature of thecylindrical desolvation chamber is room temperature or below in thecold-spray ionization mode.

In another feature of the present invention, the temperature of thecylindrical desolvation chamber is from room temperature to about 0° C.in the cold-spray ionization mode.

In another feature of the present invention, a movable desolvationchamber has a direction-changing channel. Liquid droplets are introducedfrom the opening on the side of the nebulization nozzle and passedthrough the channel to the exit opposite to the sampling orifice. Then,the sample ions are discharged.

In another feature of the present invention, the second desolvationchamber is supported by a thin support rod for heat insulation.

In another feature of the present invention, the movable desolvationchamber is fitted with temperature control means, such as a microheater,Peltier element, and sensor.

In another feature of the present invention, the potential differencebetween the movable desolvation chamber and the sampling orifice is fromzero to hundreds of volts.

In another feature of the present invention, where ions to be observedare positive ions, the potential at the sampling orifice is set lower.Conversely, where ions to be observed are negative ions, the potentialat the sampling orifice is set higher.

In another feature of the present invention, the temperature of thesampling orifice is set to approximately +80° C. in the electrosprayionization mode and to approximately room temperature in the cold-sprayionization mode.

In another feature of the present invention, the amount of sample ionsproduced in the cold-spray ionization mode is from one-hundredth toone-thousandths ( 1/100 to 1/1,000) of the amount of sample ionsproduced in the electrospray ionization mode.

Other objects and features of the invention will appear in the course ofthe description thereof, which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing illustrating the relationship between anion source and associated mass spectrometer;

FIG. 2A is a diagram of an electrospray ion source for a massspectrometer according to the present invention in which thespectrometer is operated in the ESI mode; and

FIG. 2B is a diagram of the ion source of FIG. 2A in which thespectrometer is operated in the cold-spray ionization mode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates the relationship between an ion source 20 and a massspectrometer 30. The ion source 20 supplies the ions which thespectrometer 30 separates by mass-to-charge ratio as is well understoodin the art. Often, the combination of the ion source 20 and massspectrometer 30 is simply referred to as a spectrometer.

An electrospray mass spectrometer according to one embodiment of thepresent invention is shown in FIGS. 2A and 2B. FIG. 2A shows the mannerin which the instrument is operated in the ESI mode. FIG. 2B shows themanner in which the instrument is operated in the cold-spray ionizationmode. The novel electrospray mass spectrometer is a single instrumentcapable of both analysis in the ESI mode and analysis in the cold-sprayionization mode.

FIG. 2A illustrates the usage in the ESI mode. A sample inlet port 1consists of a pipe fitted with a joint. A solution sample is supplied tothe sample inlet port 1 from a syringe pump (not shown) and is guidedinto a capillary 3. A pipe 4 coaxially surrounds the outer surface ofthe capillary 3. A room-temperature nebulizing gas consisting of aninert gas, such as nitrogen gas, is admitted from the gas inlet port 2into the pipe 4. The front end of the pipe 4 forms a nebulizationnozzle. The front end of the nozzle is inserted in a cylindrical firstdesolvation chamber 5 substantially coaxially and opens into thischamber. A heater 13 (shown schematically) for heating is buried in theinner wall of the desolvation chamber 5. A power supply (not shown)applies a potential difference of about 1 to 3 kV between the inner wallof the cylindrical first desolvation chamber 5 and the nebulizationnozzle. Because of this potential difference, a mixture of droplets ofthe sample and the nebulizing gas is electrostatically sprayed from thefront end of the nebulization nozzle. At this time, the flow rate of thesolution sample is about 1 to 1,000 microliters per minute. Aheating-and-drying gas heated to about +100 to 300° C. is admitted intothe first desolvation chamber 5 from a gas inlet port 6. The inner wallof the first desolvation chamber 5 is heated to about +100 to 300° C. bythe heater 13. Thus, radiative heat is produced from this inner wall.The heating-and-drying gas and the radiative heat cooperate to vaporizethe solvent molecules in the sample droplets. Consequently, the liquiddroplets are dried and desolvated.

A support rod 8 extends from a control knob 7. A second movabledesolvation chamber 9 is mounted at the front end of this support rod 8.This movable desolvation chamber 9 is used in the cold-spray ionizationmode. During the ESI mode, the second desolvation chamber 9 is retractedand placed off the axis of the nebulization nozzle and so desolvatedsample molecular ions fly toward a sampling orifice 10 without beingguided by the second desolvation chamber 9. The orifice 10 is heated toabout +80° C.

The space between the sampling orifice 10 and a skimmer orifice 11 isevacuated to about 200 Pa by a rotary vacuum pump (not shown). Theinside of the skimmer orifice 11 is evacuated to a higher degree ofvacuum of about 1 Pa. Therefore, the desolvated sample molecular ionsare sucked from the sampling orifice 10 into the skimmer orifice 11 andpassed into an analyzer chamber, which is maintained at a high vacuum ofabout 10⁻³ Pa, through an ion guide 12.

The potential difference between the sampling orifice 10 and thenebulization nozzle is set to about 1 to 3 kV. The potential differencebetween the sampling orifice 10 and the first desolvation chamber 5 isset from 0 to hundreds of volts. Where ions to be observed are positiveions, the potential at the sampling orifice 10 is set lower. Conversely,where ions to be observed are negative ions, the potential at thesampling orifice 10 is set higher.

FIG. 2B shows the usage in the cold-spray ionization mode. In this mode,the operator pushes in the control knob 7 to move and set the seconddesolvation chamber 9 into the position on the axis of nebulizationnozzle. Liquid droplets sprayed from the front end of the nebulizationnozzle are guided into the second desolvation chamber 9.

The sample solution is introduced into the capillary 3 through thesample inlet port 1. The nitrogen gas cooled from room temperature toabout −50° C., more preferably, from room temperature to −10° C., isintroduced from the gas inlet port 2 into the pipe 4 that coaxiallysurrounds the outer surface of the capillary 3. The capillary 3 and pipe4 together form a nebulization nozzle. The front end of the nebulizationnozzle is inserted in the cylindrical first desolvation chamber 5substantially coaxially and opens into this chamber.

Because of the potential difference of about 1 to 3 kV applied betweenthe inner wall of the cylindrical first desolvation chamber 5 and thenebulization nozzle by the power supply (not shown), a mixture ofdroplets of the sample solution and cooled nitrogen gas areelectrostatically sprayed from the front end of the nozzle or aresprayed while no voltage is applied. Under this state, the flow rate ofthe solution sample is set to 1 to 1,000 microliters per minute. At thistime, the introduction of the heating-and-drying gas into the gas inletport 6 is normally discontinued to prevent the liquid droplets frombeing warmed. Instead of the heating-and-drying gas, a low-temperature,drying gas that is controlled to cool may be supplied.

In this mode, the heater 13 buried in the inner wall of the firstdesolvation chamber 5 is deenergized and so no heating is done.Therefore, room temperature or below is maintained. The desolvationfunction is not performed. Removal of the solvent from the sprayedliquid droplets is reduced to a minimum. Only the function of producingmultiply charged molecular ions with solvent molecules attached isimplemented.

Then, the low-temperature liquid droplets are passed into the seconddesolvation chamber 9 and collided against the chamber wall togetherwith the low-temperature nebulizing gas, the second desolvation chamber9 being cooled from room temperature to about 0° C. by the coolingnebulizing gas itself. During the process where the droplets passthrough the direction-changing channel, they are pulverized minutely.The solvent is partly vaporized off without heating the liquid droplets.The amount of the resulting sample molecular ions is 1/100 to 1/1,000compared with the case of the ordinary ESI process. Hence, theanalytical sensitivity for the sample concentration is not as good.However, molecular structures of the sample molecular ions that would beeasily destroyed by the ordinary ESI process using heating aremaintained due to the low temperature.

The second desolvation chamber 9 is so designed that the liquid dropletsare admitted from the opening on the side of the nebulization nozzle.The droplets pass through the direction-changing channel. The samplemolecular ions are discharged from the exit opposite to the samplingorifice 10. Therefore, it is essential to finely adjust the position ofthe opening of the second desolvation chamber 9 relative to thenebulization nozzle. Thus, an XY manipulator is provided to permit anoptimum position to be searched for by finely adjusting the surfaceagainst which the spray is collided.

This second desolvation chamber 9 is supported by a thin support rod 8to maintain the low temperature. This prevents external heat fromentering from the control knob 7 through the support rod 8.Consequently, this support rod 8 acts as a heat insulation material.

Sample ions emerging from the second desolvation chamber 9 are drawninto the sampling orifice 10, which is pumped down to about 200 Pa by arotary pump (not shown), and then into the skimmer orifice 11 that isevacuated to about 1 Pa. Subsequently, the ions are passed via the ionguide 12 into the analytical chamber that is maintained at a high vacuumof about 10⁻³ Pa.

In the cold-spray ionization mode, the temperature of the samplingorifice 10 is kept close to room temperature by deenergizing the heater.

In the cold-spray ionization mode, the potential difference between thesampling orifice 10 and the nebulization nozzle is set to about 1 to 3kV and the potential difference between the orifice 10 and the firstdesolvation chamber 5 is set from zero to hundreds of volts, in the sameway as in the ESI mode. The potential difference set up between thesampling orifice 10 and the second desolvation chamber 9 only in thecold-spray ionization mode is set from zero to hundreds of volts. Whereions to be observed are positive ions, the potential at the samplingorifice 10 is set lower. Conversely, where the ions to be observed arenegative ions, the potential at the sampling orifice 10 is set higher.

The set temperatures of the various portions in the ESI and thecold-spray ionization modes are listed in Table I.

TABLE I The set temperatures of the various portions ESI Mode Cold-SprayIonization Mode nebulizing Gas room temperature room temperature~−50° C.first desolvation 100~300° C. ~room temperature chamber heated dry gas100~300° C. disuse (or use cooled dry gas) second desolvation disuseroom temperature~0° C. chamber sampling orifice 80° C. ~room temperature

The set potential differences between various portions are listed inTable II.

TABLE II The set potential differences between nebulization nozzle and 1kV~3 kV sampling orifice between first desolvation chamber andzero~several hundred V sampling orifice between second desolvationchamber and zero~several hundred V sampling orifice

It is to be understood that the present invention is not limited to theabove embodiment. Rather, various changes and modifications arepossible. For example, the control knob for the second desolvationchamber 9 used in the cold-spray ionization mode is not limited to thetype in which it is operated from the side opposite to the samplingorifice 10. The control knob 7 may be operated from any side ordirection. In summary, the control knob 7 may be mounted at any desiredposition as long as this desolvation chamber can be placed off the axisof the nebulization nozzle in the ESI mode and set on the axis ofnebulization nozzle in the cold-spray ionization mode. That is, in thecold-spray ionization mode, sprayed liquid droplets can be accepted, andthe desolvated molecules can be discharged toward the sampling orifice10.

Furthermore, an adjustment may be made to optimize the positionalrelation between the nebulization nozzle and the second desolvationchamber 9, for example, by (1) moving and setting the second desolvationchamber 9 onto the axis of the nebulization nozzle and moving thisnebulization nozzle, (2) moving both second desolvation chamber 9 andthe nebulization nozzle, (3) visually checking the flow of the sprayedliquid droplets, or (4) monitoring the intensities of mass spectraobtained by the mass spectrometer from the viewing screen of thespectrometer.

The angle formed between the axis of the nebulization nozzle and theaxis of the opening of the sampling orifice 10 is set to 90° in theembodiment of FIGS. 2A and 2B. This angle is not limited to 90°. Forinstance, the angle may be varied to any desired value within the rangefrom 0° to 90°. In this case, it is only necessary that the liquiddroplets sprayed from the nebulization nozzle be taken into the seconddesolvation chamber 9 and the desolvated molecules be discharged towardthe sampling orifice 10.

Moreover, the second desolvation chamber 9 may have a built-inmicroheater, Peltier element, sensor, or other temperature control meansto provide an accurate temperature control.

Further, the exit opening of the second desolvation chamber 9 is notalways required to be coaxial with the opening of the sampling orifice10.

As described so far, the present invention makes it possible to performmeasurements consecutively from the ESI mode to the cold-sprayionization mode and vice versa by simply pushing or pulling the controlknob. It is not necessary to prepare two ion sources. Consequently, thecost can be reduced. In addition, in the ESI mode, the desolvationchamber for the cold-spray ionization mode operation is retracted and socontamination due to adhesion of liquid droplets is low. Hence, it iseasy to perform cleaning.

Having thus described our invention with the detail and particularityrequired by the Patent Laws, what is desired to be protected by LettersPatent is set forth in the following claims.

1. An electrospray mass spectrometer fitted with an electrospray ionsource, said ion source comprising a structure supporting a nebulizationnozzle, a sampling orifice, a heated desolvation chamber and a controlknob supporting a support rod, said nebulizing nozzle having an axis andsaid sampling orifice having an axis, the axis of the nebulizationnozzle intersecting the axis of the sampling orifice, said electrosprayion source further comprising a movable desolvation chamber having adirection-changing channel and being supported from said support rodsuch that the movable desolvation chamber can be moved off the axis ofthe nebulization nozzle in an electrospray ionization mode and set onthe axis of the nebulization nozzle in a cold-spray ionization modewherein liquid droplets are introduced from an opening from thenebulization nozzle and pass through the direction-changing channel suchthat sample ions are discharged from an exit opposite to the samplingorifice.
 2. The electrospray mass spectrometer of claim 1, wherein saidnebulization nozzle consists of a capillary for guiding a samplesolution supplied from a sample inlet port and a pipe for guiding anebulizing gas introduced from a gas inlet port, said pipe coaxiallysurrounding the outer surface of said capillary.
 3. The electrospraymass spectrometer of claim 2, wherein the temperature of said nebulizinggas is adjustable between room temperature and approximately −50° C. Foruse of the electrospray mass spectrometer in the cold-spray ionizationmode.
 4. The electrospray mass spectrometer of claim 1, wherein saidheated desolvation chamber is cylindrical and said nebulization nozzleis substantially coaxially inserted in a heated cylindrical desolvationchamber, the nozzle opening into the cylindrical desolvation chamber,and wherein said cylindrical desolvation chamber has a gas inlet portfor introducing a heating-and-drying gas.
 5. The electrospray massspectrometer of claim 4, wherein a potential difference of 1–3 kV isimposed between said nebulization nozzle and the sampling orifice, andwherein a potential difference from zero to hundreds of volts is imposedbetween said cylindrical desolvation chamber and the sampling orifice.6. The electrospray mass spectrometer of claim 4, wherein in theelectrospray ionization mode, the heating-and-drying gas is introducedinto said cylindrical desolvation chamber from the gas inlet port, andwherein the introduced heating-and-drying gas and heating performed by aheater buried in an inner wall of the desolvation chamber cooperate todry and desolvate the liquid droplets.
 7. The electrospray massspectrometer of claim 6, wherein the heater for the cylindricaldesolvation chamber is controllable between 100 and 300° C.
 8. Theelectrospray mass spectrometer of claim 6, wherein the temperature ofsaid heating-and-drying gas is controllable between 100 and 300° C. 9.The electrospray mass spectrometer of claim 4, wherein means for cuttingoff the supply of the heating-and-drying gas from the gas inlet port andmeans to deenergize the heater buried in the inner wall of thecylindrical desolvation chamber are provided to avoid heating of theliquid droplets passing therethrough in the cold-spray mode.
 10. Theelectrospray mass spectrometer of claim 9, wherein means are providedfor supplying a cooled gas into said cylindrical desolvation chamberfrom the gas inlet port.
 11. The electrospray mass spectrometer of claim9 or 10, wherein temperature of said movable desolvation chamber issetable to room temperature or below in the cold-spray ionization mode.12. The electrospray mass spectrometer of claim 1, wherein when amixture of droplets of a sample and a nebulizing gas areelectrostatically sprayed from said nebulization nozzle and the flowrate of sample solution is setable to 1–1,000 microliters per minute.13. The electrospray mass spectrometer of claim 1, further comprisingmeans for setting the temperature of said desolvation chamber betweenroom temperature and approximately 0° C. in the cold-spray ionizationmode.
 14. The electrospray mass spectrometer of claim 1, wherein saidmovable desolvation chamber is supported by a thin support rod for heatinsulation.
 15. The electrospray mass spectrometer of claim 1, whereinsaid movable desolvation chamber is fitted with temperature controlmeans such as a microheater, Peltier element, or sensor.
 16. Theelectrospray mass spectrometer of claim 1, wherein a potentialdifference of zero to hundreds of volts is developed between saidmovable desolvation chamber and said sampling orifice.
 17. Theelectrospray mass spectrometer of claim 1, further comprising means forsetting the sampling orifice to a temperature of approximately +80° C.in the electrospray ionization mode and to around room temperature inthe cold-spray ionization mode.
 18. The electrospray mass spectrometerof claim 1, wherein the ratio of the amount of ions relative to sampleconcentration produced in the cold-spray ionization mode is 1/100 to1/1,000 of the amount of ions relative to sample concentration producedin the electrospray ionization mode.
 19. The electrospray massspectrometer of claim 1, wherein the direction-changing channel in themovable desolvation chamber is configured to pulverize the liquiddroplets minutely to cause partial vaporization without heating thedroplets.